1. Field of the Invention
The present invention hereinafter disclosed relates to a light-emitting element having a light-emitting layer between a pair of electrodes. In addition, the present invention relates to a light-emitting device in which the light-emitting element is used and a lighting device and an electronic device in each of which the light-emitting device is used.
2. Description of the Related Art
In recent years, a light-emitting element in which a light-emitting organic compound or a light-emitting inorganic compound is used as a light-emitting material has been actively developed. In particular, a light-emitting element called an electroluminescence (hereinafter, EL) element has a simple structure in which a light-emitting layer containing a light-emitting material is provided between electrodes, and has attracted attention as a next-generation flat panel display element because of its characteristics such as thinness, lightweight, high response speed, and direct current low voltage driving. In addition, a display in which such a light-emitting element is used has a feature that it is excellent in contrast and image quality and has a wide viewing angle. Moreover, such a light-emitting element is a plane light source; therefore, application of such a light-emitting element is considered as a light source such as a backlight of a liquid crystal display and lighting.
Current is applied to a light-emitting layer provided between a pair of electrodes in a light-emitting element to excite a light-emitting material contained in the light-emitting layer, whereby a predetermined emission color can be obtained. Supplying a large amount of current to the light-emitting layer is considered in order to increase emission luminance of such a light-emitting element; however, such a method hinders reduction of power consumption. In addition, applying a large amount of current also leads to acceleration of deterioration of the light-emitting element.
Hence, a light-emitting element is proposed whose emission luminance is increased by stacking a plurality of light-emitting layers and applying current which has the same current density as current applied in the case of a light-emitting element having a single light-emitting layer (for example, Patent Document 1).
[Reference]
[Patent Document]
    [Patent Document 1] Japanese Published Patent Application No. 3933591
In Patent Document 1, a light-emitting element is proposed in which a plurality of light-emitting units (hereinafter in this specification, the light-emitting unit is also referred to as an EL layer) are provided and the light-emitting units are separated by a charge production layer. More specifically, a light-emitting element is proposed in which a charge production layer formed of vanadium pentoxide is provided over a metal-doped layer functioning as an electron-injecting layer of a first light-emitting unit, and further a second light-emitting unit is stacked over the metal-doped layer with the charge production layer interposed therebetween. However, in the light-emitting element having such a structure, mutual interaction occurs between the metal-doped layer and the charge-generation layer formed of the oxide at their interface and the interface has a high electric field; thus, high voltage is needed for driving the light-emitting element, unfortunately.
In view of the above problem, it is an object to provide a light-emitting element which exhibits light emission with high luminance and can be driven at low voltage. In addition, it is another object to provide a light-emitting device or an electronic device with reduced power consumption.